Mars vs. Europa: Are we looking in the wrong place for alien life?

A British astrobiology conference has revived a years-old debate over the best place to look for life elsewhere in the solar system: Mars, or the moons of Jupiter and Saturn?

"For reasons I don't really understand, the wider solar system and the potential for life there has not been high priority," The Telegraph quoted Robert Pappalardo, a senior research scientist at NASA's Jet Propulsion Laboratory, as saying on BBC Radio 4.

Pappalardo's remarks were occasioned by this week's astrobiology conference at the UK Center for Astrobiology in Edinburgh, Scotland. The center recently established the International Subsurface Astrobiology Laboratory, or ISAL, half a mile (1 kilometer) beneath the surface in Yorkshire's Boulby mine. Biologists will use that facility to see how organisms hold up in extreme environments, learn about life's chemical signatures, and test instruments that could look for those signatures on other worlds.

If Europa's ocean is 100 kilometers (62 miles) deep, and all that water were gathered into a ball, it would have a radius of 877 kilometers (545 miles). This graphic compares that hypothetical ball of Europan water to the size of the Jovian moon itself, as well as all the water on planet Earth. Europa is thought to have two to three times the volume of water in Earth's oceans.

At February's annual meeting of the American Association for the Advancement of Science, Pappalardo worried that NASA's study of the outer solar system would go "radio-dark" in 2017, when the Cassini mission to Saturn and the Juno mission to Jupiter are both due to end. He continued that theme in this week's BBC interview.

"I worry that if Europa exploration is delayed, but then finally it happens some day, we might look back and say 'Why didn't we do that sooner?' Imagine 50 years from now, we get a lander there and find signs of life. All this time we'll have been looking in the wrong place," he was quoted as saying.

Europa isn't the only moon that intrigues astrobiologists: In the Jovian system, Callisto and Ganymede also have icy shells and may hold hidden oceans. Meanwhile, Cassini has repeatedly observed geysers of water ice rising from the surface of the Saturnian moon Enceladus — suggesting that liquid water and perhaps life may lie beneath the surface. Saturn's largest moon, Titan, has a thick atmosphere and seas of hydrocarbon that some scientists think could harbor a totally alien kind of life.

So what if all of these worlds — Mars and Europa, Callisto and Ganymede, Titan and Enceladus — turn out to be lifeless? Charles Cockell, who heads the UK Center for Astrobiology, addressed that scenario in an interview with the BBC.

"A lot of people think astrobiology is some sort of hunt for life, and if we don't find life, it will be a big disappointment," Cockell said. "But in fact, that's not the case. The discovery of many lifeless planets across the universe, the discovery that the Earth might be unique as a place for life, would be an astonishing discovery in itself. It would be a very lonely discovery, but it would be an astonishing discovery."